Optical limiters are used for the protection—for instance of eyes, of photodetectors, or of cameras—against unexpected strong illumination. The present invention provides an optical power limiting device, primarily for protection in the LWIR region. The optical limiter of this invention is transparent under a low level of illumination at the wavelength of interest, but is “dark” under strong incident light power. For incident power that exceeds the power threshold of the present optical limiter, the optical signal power transmitted through the optical limiter is substantially constant (at the threshold level), no matter what the incident power is. Optical limiters of the present invention have low initiating thresholds and broad spectral ranges.
Optical limiting devices have been made with Reverse Saturable Absorber (RSA) solutions and multi-photon absorber dyes. Such materials absorb more light as the intensity of the incident light increases. A disadvantage of such materials arises from their use of nonlinear optical absorption processes. This leads to a high threshold for the optical limiting behavior to switch on. More importantly, unlike the present invention, conventional RSA or multi-photon solutions cannot be used in the LWIR region.
The use of vanadium oxide interference mirrors for optical limiting in the LWIR region has been proposed. See Konovalova et al., “Interference systems of controllable mirrors based on vanadium dioxide for the spectral range 0.6-10.6 μm”, J. Opt. Technol., 66 (5):391-398 (1999). The device proposed by Konovalova et al. would use vanadium oxide film to absorb incident laser energy and change phase. A primary function of the Konovalova limiter is laser hardening. It is therefore a relatively narrowband device that is not suitable for handling a broadband range of wavelengths. The Konovalova approach requires an initiating threshold on the order of 1 MW/cm2, which is too high for many applications. A mid-IR and LWIR range limiter, for instance, must limit a continuous wave having a broad band but having relatively low peak power radiation (about 1 W/cm2).
It is well known that vanadium dioxide (VO2) thin films experience phase change from semiconductor to metal at around 68° C. Since this phase change is solid to solid, the phase change speed can be as fast as less than 150 femtoseconds. In the phase change process, the refractive index of the VO2 varies dramatically. For example, at a wavelength of 10.6 microns, the refractive index of VO2 varies from 2.55-0.08i in the semiconductor phase to 8-9i in the metal phase. At the different refractive indexes, the electromagnetic radiation is transmitted through or reflected by the film.
FIG. 1A, taken from Verleur et al., Physics Review, 172(3):788-798 (1968), depicts the temperature dependence of optical transmission at 0.31 eV and resistivity of a 1000 Å film of VO2. FIG. 1A shows that the transmission through the film can vary from 90% to 10% as temperature increases from 60° C. to 70° C.
As illustrated in FIG. 1B, taken from Barker et al., Physics Review Letters, 17(26):1286-1289, the variation of refractive index or transmission through the film is broadband, from near-IR to mid-IR to LWIR. In FIG. 1B, the solid curve is a fit for T<Tt using eight phonon modes and one band-structure mode. The crosses and squares show the data above Tt. The squares are low because of sample cracking. However, they illustrate the monotonic rise expected of free carrier reflection. The triangle points were taken upon cooling. Because of thermal hysteresis, Tt≈63° C. for this run. The present invention makes use of this variable characteristic of the film to provide a broadband optical limiting device.
An object of the present invention is to protect detector devices such as LWIR cameras against damage from continuous wave broadband radiation, such as sunlight. Because of its high temperature and brightness, continuous wave radiation can be more harmful to a detector such as a camera than a pulsed narrow bandwidth laser would be. On the other hand, because of its high temperature, the continuous wave radiation has hundreds of times more energy in the visible and near infrared spectrum than in the long wave infrared region. The present invention makes use of the energy from visible and near infrared radiation to heat and trigger the optical limiting function of the present optical limiter devices.